After Selecting Listview App is Crashed

Adding item after select list view select app is crashed
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Code Behind
List ss;
public AddMenuPage()
{
InitializeComponent();
ss =new List() { "one", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine" };
SelectionListView.ItemsSource = ss;
}

    private void Button_Clicked(object sender, EventArgs e)
    {
        ss.Add("Ten");     
        SelectionListView.ItemsSource = ss;
    }